This invention relates to a method of forming metal colloids and metal colloids. In particular, though not exclusively, the present invention relates to a method of forming a metal colloid for use in forming a metal oxide sensitive layer of a chemical sensor device.
A metal oxide chemical sensor device, like a tin oxide chemical sensor device, comprises a metal oxide sensitive layer whose resistance varies when exposed to oxidising or reducing chemicals. The selectivity of the device to certain chemicals depends on the temperature at which the sensitive layer is maintained. Thus, by measuring the change in the resistance of the sensitive layer and the temperature of the sensitive layer, the concentration of a particular chemical can be determined. The well known theory of operation of these devices involves an adsorption/desorption phenomena at the surface of the sensitive layer, which is crystalline. This is explained in an article by N. Yamazoe in Sensors and Actuators B, 5, 1991, pages 7-19.
The sensitivity of such a sensor can be significantly improved by reducing the size of the metal oxide crystals which form the sensitive layer. It is therefore desirable to use small metal particles to form the metal oxide sensitive layer. These small particles are less than 0.1 micron in size, i.e. are nano sized particles, and are generally referred to as nano particles.
For tin oxide chemical sensors, it is known to use cathodic sputtering of a metallic target to obtain small tin particle. This sputtering process leads to the deposition of metallic tin particles, which particles are later oxidised into tin oxide by temperature treatment under air in a furnace. The tin particles each have a thickness 0.16-0.19 microns. FIG. 1 is a Scanning Electron Microscope (SEM) micrograph of tin sputtered particles (at a magnification of xc3x9716553). Due to the columnar growth of the sensitive layer, which is inherent with cathodic sputtering, the particle size and sensitive layer thickness are strongly interdependent: that is, the thicker the sensitive layer, the larger the particle size. Thus, although the sputtering process ensures a well controlled particle size and also thickness of the tin oxide sensitive layer, due to the interdependence between the grain size and sensitive layer thickness, there is a limit below which the particle size cannot be reduced. In other words, it is possible to deposit by sputtering very small nano particles but in this case, the process is not under control and leas to too thin metal layers for chemical sensor applications. Details of the sputtering technique have been published in an article by V. Demarne and A. Grisel in xe2x80x98Sensors and Actuatorsxe2x80x99, B, 15-16 (1993) pages 63-67.
Other techniques to provide small metal particles or metal nano particles have also been explored.
An article by A. Henglein and M. Giersig in the J. Phys. Chem. 1994, 98, pages 6931-6935, describes preparing tin nano particles by radiolytic reduction of tin chloride (SnCl2) using gamma radiation from a cobalt source (60Co).
From an article in the Colloid and Polymer Science 1994, pages 272, 310, by G. Cardenas-Trino, M. Alvial, K. J. Klabunde, M. O. Pantoja, Z. H. Soto and from an article by E. Sondergard, R. Kofman, P. Cheyssac, A. Stella in the Applied Surface Science, 1996, pages 364, 467, it is also known that tin particles, which may or may not be stabilised by a polymer, can be prepared by evaporation condensation methods, either by Chemical Liquid Deposition (CLD) to yield particles having sizes in the range 15-50 nm codeposited with a solvent at 77 Kelvin, or by metal evaporation under ultra high vacuum and condensation leading to various sizes of particle (in the range of 1-150 nm), the size being a function of the growth mode.
All these solutions, however, require heavy and costly equipment, such as a radioactive source, ultra low temperature equipment, and thus cannot be realistically used on an industrial scale, for example, in the manufacture of semiconductor chemical sensors.
There is therefore a need for an improved method of forming metal nano particles.